The exchange of gases in the lung requires ventilation, perfusion and the diffusion of gases across the blood-gas barrier of the alveoli. While pulmonary ventilation (1, 2) and perfusion (3, 4) can be examined by a variety of imaging techniques, currently no methods exist to image alveolar-capillary gas transfer. Unfortunately, certain pulmonary pathologies such as, for example, inflammation, fibrosis, and edema may initially have a predominant effect on the gas exchange process, but not ventilation or perfusion. The degree to which a “diffusion block” (5) is present or absent in the blood-gas barrier has been difficult to determine in studies to date (6). In healthy alveoli, the harmonic mean thickness [as defined by Weibel (7) of the blood-gas barrier is about 0.77 μm and oxygen traverses this space in less than a millisecond, saturating the red blood cells (RBCs) in tens of milliseconds. However, in regions where the barrier is thickened, oxygen may be undesirably prevented from diffusing across the barrier fast enough to saturate the RBCs before they exit the gas exchange region [estimated at about 750 ms in humans (5), 300 ms in rats (8).